Rotary fluid film face seals, also called gap or non-contacting face seals, are usually applied to high-speed and/or high-pressure rotating equipment wherein the use of ordinary mechanical face seals with face contact would result in excessive heat generation and wear. Non-contacting operation avoids this undesirable face contact at times when the shaft is rotating above a certain minimum speed, which is called a lift-off speed.
There are various ways of accomplishing the above non-contacting operation. One of the more commonly used ways includes the formation of a shallow angled (i.e. spiral) groove pattern in one of the sealing faces. The sealing face opposite the grooved face is relatively flat and smooth. The face area where these two sealing faces define a sealing clearance is called the sealing interface.
The above-mentioned angled groove pattern on one of the sealing faces normally extends inward from the outer circumference and ends at a particular face diameter called the groove diameter, which is larger then the inner diameter of the seal interface. The non-grooved area between the groove diameter and the inner interface diameter serves as a restriction (i.e. a dam) to fluid outflow. Fluid delivered by the groove pattern must pass through this restriction and it can do so only if the sealing faces separate. The way this works is through pressure build-up. Should the faces remain in contact, fluid will be compressed just ahead of the restriction, thus building up pressure. The pressure causes a separation force which eventually becomes larger than the forces that hold the faces together. In that moment the sealing faces separate and allow the fluid to escape. During operation of the seal, an equilibrium establishes itself between fluid inflow through angled groove pumping and fluid outflow through face separation. Face separation is therefore present as long as the seal is operating, which means as long as one face is rotating in relation to the opposite face.
However, angled groove pumping is not the only factor that determines the amount of the separation between the sealing faces. Just as the angled grooves are able to drive the fluid into the non-grooved portion of the sealing interface past the groove diameter, so can the pressure differential. If enough of a pressure difference exists between the grooved end of the interface and the non-grooved end, fluid will also be forced into the non-grooved portion of the interface, thereby separating the faces and forming the clearance.
Both ways in which clearance can be formed between the sealing faces, one with speed of rotation, the other with pressure differential, are distinct and separate, even though the effects of both combine on the operating seal. If there is no pressure difference and the seal face separation occurs strictly due to face rotation, forces due to fluid flow are hydrodynamic if the fluid sealed is a liquid, and aerodynamic if the fluid sealed is a gas.
On the other hand, if there is no mutual rotation between the two sealing faces and face separation is strictly the consequence of pressure differential between both ends of the sealing interface, forces due to fluid flow are hydrostatic if the fluid sealed is a liquid, and aerostatic if the fluid sealed is a gas. In the following, the terms hydrostatic and hydrodynamic are used for both liquid and gas effects since these latter terms are more conventionally used when describing both liquid and gas seals.
A typical spiral groove seal needs to provide acceptable performance in terms of leakage and the absence of face contact during all regimes of seal operation. It must do so not only at top speed and pressure, but also at standstill, at start-up, during acceleration, at periods of equipment warm-up or at shutdown. At normal operating conditions, pressure and speed vary constantly, which results in continuous adjustments to the running clearance. These adjustments are automatic; one of the key properties of angled groove seals is their self-adjustment capability. On change in speed or pressure, the face clearance adjusts automatically to a new set of conditions. However, the operating envelope of speeds and pressures is usually very wide and a seal design of necessity must be a compromise. For its performance to be acceptable at near-zero speed or pressure, it is less than optimum at operating speed and pressure. This is simply due to the fact that, both in terms of pressure and speed, the seal has to be brought up to operating conditions from zero speed and zero pressure differential.
While known fluid seals have attempted to provide both hydrodynamic and hydrostatic sealing properties, nevertheless the known seals have still been deficient with respect to their ability to optimize these properties.
More specifically, in known groove patterns, the angled grooves have typically been formed such that the width ratio between the grooves and the lands adjacent the grooves in a circumferential direction remains relatively constant as the grooves angle inwardly from the outer diameter. This thus necessarily results in the flat intermediate face or land area between adjacent grooves being of decreasing area (i.e., width) as these lands angle inwardly from the outer diameter. This progressive decrease in the width of the land area as the land angles inwardly from the outer diameter decreases the squeeze film effect in the fluid which flows over the land during operation, and thus decreases the thrust bearing support the lands provide with respect to avoiding seal face contact under conditions of high speed rotation.
Accordingly, it is on object of this invention to provide an improved fluid seal of the type employing a groove pattern on one of the opposed seal faces, which groove pattern includes a series of angled grooves formed in and positioned circumferentially around one of the seal faces, with the configuration of the grooves and of the flat face lands defined between adjacent grooves being configured so as to improve the squeeze film effect over the lands and between the opposed faces to thus provide improved hydrodynamic sealing characteristics, particularly under conditions of high speed.
More specifically, in the improved seal of this invention, the angled grooves are preferably formed such that the sides of adjacent grooves extend generally in parallelism with one another so that the intermediate land area between adjacent grooves maintains a substantially constant width, even adjacent the radially inner ends of the grooves, to maximize squeeze film effects in the fluid which flows over the lands to thus enhance the thrust bearing support these lands provide for avoidance of seal face contact at or near high speed of rotation.
Other objects and purposes of the invention will be apparent to persons familiar with seals of this general type upon reading the following specification and inspecting the accompanying drawings.